The goal of this research is to elucidate the function of histone acetylation. Acetylation of lysine residues on the amino termini of the core histones has long been considered a mechanism for modulation of changes in chromatin structure. It has been proposed that neutralization of these positive charges would decrease the affinities of histone amino termini for DNA and allow unfolding of selected chromatin regions for DNA replication, repair and transcription. Support for this hypothesis is mainly correlative. Thus it is essential to test for the involvement of histone acetylation in these processes by observing the in vivo effects of manipulating the levels of histone acetylation. The yeast Saccharomyces cerevisiae represents an ideal system for this analysis since histone acetylation can be reduced by deletions or mutations in the histone genes and in the gene(s) for histone acetyltransferase enzyme(s). An analysis of chromatin structure and transcriptional activity in yeast strains carrying mutations in the amino terminus of histone H2B is proposed. The sites of acetylation will be determined for wild type and mutant cell H2B proteins. Comparisons of the internal nucleosome structure and higher orders of chromatin folding will be made. In addition, levels of transcription of a variety of genes will be measured for H2B mutant and wild type cells. To investigate the changes when acetylation levels of all four yeast histones are reduced, mutants with defective histone acetyltransferase will be produced (HAT mutants). The cell cycle effects of these HAT mutations will be observed. Chromatin structure, DNA repair, replication, chromatin assembly and transcriptional activity will be compared for HAT mutant and wild type strains. Yeast represents a model system that is highly suited to this analysis and the results obtained here will relate to the expression of genes in higher organisms that are not amenable to this genetic manipulation. In particular, expression is essential or to aberrant processes such as neoplasia where programmed gene expression is altered.